An instrument for measuring characteristics of a particle sample by counting and classifying particles into selected ranges of particle characteristics. The particle concentration is reduced to the level where the probability of measuring scattering from multiple particles of interest at one time is reduced to an acceptable level. A light beam is projected through a sample space, through which the particles flow. As each particle passes through the beam, it scatters, absorbs, and transmits different amounts of the light, depending upon the particle characteristics. So both the decrease in the beam intensity, due to light removal by the particle, and light scattered by the particle, may be used to determine the particle characteristics, to classify the particle and count it in a certain range of particle characteristics.